Surveying apparatuses or scanners of the type mentioned in the introduction are designed to survey a spatial region and/or an object three-dimensionally. Typical applications include, for example, the surveying of interiors, such as churches and factory buildings, large objects, such as buildings or aircraft, or else the forensic surveying of an accident site.
In order to survey these objects, by means of a laser scanner, a predefined spatial volume is scanned with a laser beam and the laser light reflected from the object is detected, wherein the angle information for the direction of the emitted laser beam and of the detected laser light is acquired for each instant. By means of the detected laser light, the distance between a surface point situated in the spatial volume and the measuring apparatus can be determined by triangulation and/or time-of-flight measurement or phase shift. Together with the angle information associated with said surface point, it is possible to calculate the spatial position of said surface point. From the sequence of measurement points recorded in this way, or the positions in space calculated therefrom, a three-dimensional model of the scanned surface, of the object or of the scanned environment is generated by corresponding software, e.g. in the form of a three-dimensional point cloud.
In many cases, besides this purely geometrical acquisition of the surfaces in the spatial volume, a photographic acquisition by means of a camera is also carried out. An overview camera simply fitted on the surveying apparatus was initially used for this purpose. The basic principles of such a scanning surveying apparatus with camera are explained in DE 20 2006 005 643 U1.
Modern surveying apparatuses of this type generally have a measuring camera arranged alongside the radiation source and the detector for the reflected radiation in the housing of the surveying apparatus or scanner (on-axis measuring camera). By means of corresponding optical deflection devices, the field of view of such a camera can be aligned for example coaxially with respect to the measurement beam optics or parallel thereto. The acquired image can be represented on a display of the display control unit of the surveying apparatus and/or on the display of a periphery device used for remote control—such as a data logger, for example. However, rotations that the captured image exhibits on account of the deflection of the camera field of view at the beam deflection unit first have to be corrected computationally prior to representation, for which reason real-time representations cannot be realized.
For capturing relatively large spatial volumes, the integrated camera generally captures a plurality of images that are subsequently combined by corresponding software to form a single image in the sense of a “panoramic image”. Such photographic images captured in parallel with the scanning allow at least the identification of different brightness or color values of the scanned surfaces. Besides the visual overall view, under certain circumstances further information, e.g. regarding the surface texture, can also be obtained therefrom.
What is disadvantageous about these surveying apparatuses is that as a result of the partly common optical path of camera and measurement radiation/reflection radiation, either the optical path of the measurement radiation/reflection radiation is impaired by constituents of the camera optics, or conversely the optical path of the measuring camera is impaired by measurement radiation/reflection radiation or optical components or the paths thereof. The optical construction of such surveying apparatuses is complicated and expensive, and indeed it requires the coupling-out of defined light components from the common optical path. Furthermore, for panoramic recordings captured by such surveying apparatuses, the individual images of which the panoramic recording is composed have to be processed by corresponding software in a complicated manner, since, by virtue of the stationary photosensitive sensor (CMOS, CCD) of the camera in interaction with the field of view rotating about two axes, the individual images are captured in a manner rotated by different angles in each case.
EP 2 620 746 A1 presents a surveying apparatus comprising a laser scanning unit for surveying and representing objects or the environment in the form of point clouds, said apparatus having an overview camera alongside an on-axis surveying camera. The overview camera has an overview field of view, which is larger than the field of view of the on-axis measuring camera, such that the area of the terrain acquired by the overview camera is larger than that acquired by the measuring camera. The overview camera therefore enables a better orientation and faster targeting of a desired point. It is therefore used primarily for the orientation in space. The overview camera can be accommodated in the measuring head and have a field of view with fixedly defined alignment in a manner similar to the overview camera from DE 20 2006 005 643 U1 already mentioned above, or else the overview camera is arranged on the exterior of the beam deflection unit and its overview field of view, like the field of view of the on-axis measuring camera, is likewise alignable by means of the beam deflection unit. What is advantageous here is that not only the field of view but also the photosensitive sensor rotates about the two axes. For this purpose, the beam deflection unit is provided with an optical channel that connects the overview camera arranged on the beam deflection unit to the rear side—embodied in a reflective fashion—of the deflection element via corresponding optical elements and enables the overview camera to have a view rotated by 180° with respect to the field of view of the on-axis measuring camera and with respect to the alignment of the measurement radiation. The construction with the overview camera on the exterior of the beam deflection unit has various disadvantages: firstly, it is necessary to provide two cameras and two optical paths for the fields of view of the cameras. Secondly, the positioning of the overview camera on the exterior of the beam deflection unit causes considerable unbalance and bending moments on the beam deflection unit, which have to be combated by corresponding countermeasures, which causes additional costs and makes the system susceptible to faults in field operation.
DE 10 2010 105027 A1 discloses a laser scanner comprising a camera, which, instead of being arranged on the exterior of the beam deflection unit, is arranged in the beam deflection unit, in the space behind the deflection mirror. An optical deflection element is additionally arranged in the space behind the deflection mirror and diverts the field of view of the camera in such a way that it is aligned from the camera lens into the environment via a window in a wall of the beam deflection unit in a manner rotated by 180° with respect to the alignment of the emitted measurement beam. Therefore, the camera always looks in exactly the opposite direction in relation to the emitted measurement beam. The manner in which the beam deflection unit and the camera are specifically configured, whether as overview camera having a large field of view or as measuring camera having a smaller field of view, and the manner in which the camera and the optical deflection element are fixed in the beam deflection unit are not disclosed in DE 10 2010 105027 A1.
Although the cameras concomitantly rotating with the beam deflection unit, as disclosed in DE 10 2010 105027 A1 and EP 2 620 746 A1, solve the problem of the rotated images that arose in the previous on-axis cameras as a result of the rotation of the camera field of view relative to the stationary camera sensor, real-time recordings are still not available to the user. The calibration of such an apparatus also remains time-consuming and has to be performed by the manufacturer. A subsequent calibration on site is virtually impossible. When determining the spatial coordinates taking account computationally of the system-inherent faults, such as tilting axis skew, etc., it is still necessary to use complex algorithms by means of which the viewing direction of measuring camera and measurement radiation rotated by 180° with respect to one another can be taken into account when determining the spatial coordinates.